The invention relates to hydraulic master-cylinders used notably in braking systems or in clutch control systems of automotive vehicles.
From French Pat. specification No. 1,440,394 hydraulic mastercylinders are known which comprise a cylinder connected to an utilizing circuit and in which a piston generally operated by a pedal through a push rod slides in a sealing-tight manner. The piston carries at both ends two sealing-tight elastic packings, one directed towards the atmosphere and one disposed with axial and radial clearance to correspond to the piston head. This elastic packing, referred to as the valve ring, cooperates with a shoulder provided on the piston head to stop communication between the pressure chamber of the cylinder which is connected to the utilizing circuit and the hydraulic fluid supply reservoir during forward movement of the piston and to restore such communication during the rearward or return phase of the piston in the event of the pressure chamber should be replenished with fluid from the reservoir.
During the return stroke of the piston assuming there was a small fluid leakage from the utilizing circuit, a vacuum can occur in the pressure chamber. This vacuum causes the valve ring to disengage the shoulder so that additional fluid enters into the pressure chamber to reduce the vacuum level.
The same type of condition frequently happens during the bleeding operation of the master-cylinder. However, it may also happen that due to friction forces between the peripheral part of the valve ring and the wall of the cylinder receiving the piston. If the valve ring prematurely disengages the shoulder during the return stroke of the piston there is a sharp decrease of the pressure level prevailing in the pressure chamber. This last event reduces the reliability of the master-cylinder by dangerously increasing the lost motion of the piston in case of vehicle braking wherein the vehicle driver operates its brake pedal by successive and relatively short actuations. According to the invention described in the before referenced patent, a spring leaf is provided to avoid premature valve-ring unseating. However, this technical solution is not satisfactory due to the fact that the spring rate should respond to two antagonist requests, on one hand being high enough to overcome the friction forces thereby avoiding premature opening and on the other hand being low enough to permit fluid replenishing as soon as vacuum is created in the pressure chamber.